The present invention relates to an organic thin-film light-emitting device for use in, for example, a flat-panel display. More specifically, the invention is directed to a structure of an organic thin-film light-emitting device for preventing electrical contact between upper electrodes in manufacturing the device and thus, resulting in high productivity.
An organic thin-film light-emitting device is also called an organic thin-film electroluminescent (hereinafter abbreviated to EL) device. The EL device emits light by recombination of an electron and a hole, which are injected to an organic thin-film by applying an electric field. The organic thin-film generally formed of multiple layers.
FIG. 3(a) and FIG. 3(b) show a conventional organic thin-film light-emitting device. FIG. 3(a) is a sectional view of a two-layered type light-emitting device, and FIG. 3(b) is a sectional view of a three-layered type. The organic thin-film light-emitting device shown in FIG. 3(a) is a two-layered type, and is constructed by sequentially forming an anode 2 which is a transparent electrode of ITO (indium tin oxide) deposited on a transparent substrate 1, an organic EL medium layer 4 formed of a hole transport layer 4a and a light-emitting layer 4b, and a cathode 5 which is a metal electrode (Appl. Phys. Lett. vol. 51, p. 913, 1987). The organic thin-film light-emitting device shown in FIG. 3(b) is a three-layered type, and further comprises an electron transport layer 4c formed between the cathode 5 and the light-emitting layer 4b. Recently, also known is a four-layered structure in which a hole injection layer is further formed between the anode 2 and the hole transport layer 4a (e.g. SID 97 DIGEST, p. 1073, 1997).
FIG. 4 is a perspective view of a conventional organic thin-film light-emitting device.
The organic thin-film light-emitting device is a so-called X, Y matrix type (simple matrix type). The device is fabricated by laminating sequentially on a transparent glass substrate 1, an anode 2 formed of a multiple of transparent electrodes made of, e.g. ITO, an electrically insulating layer 7, an organic EL medium layer 4, a metal cathode 5 which constitutes upper electrodes orthogonal to the transparent electrodes. A light-emitting area of the organic thin-film light-emitting device is composed of an organic EL medium layer 4, and a pair of anode 2 and metal cathode 5 which oppose each other and sandwich the organic EL medium layer. One pixel is formed, as one unit, of a light-emitting area formed by an overlap of a single electrode composing the transparent anode 2 and a single electrode composing the metal cathode 5 orthogonally disposed each other. A flat panel display device comprises a plurality of the light-emitting areas corresponding to the pixels, which are arranged on the substrate. A flat panel display device is operated by driving the transparent anodes 2 and the metal cathodes 5, to which lead wires (not shown) are attached around the flat panel.
Generally, patterning of the anodes 2 is performed by photolithography after forming a film of an anode material on a substrate. Namely, at first, photoresist is coated on an anode film; then, patterning of the photoresist is performed to a designed configuration by exposure and development; and finally, the anode material is etched and the photoresist is peeled off.
On the other hand, the upper electrodes are very difficult to be patterned by photolithography because the organic EL media for constituting an organic EL medium layer, such as a charge injection layer and a light-emitting layer, have a poor resistance to heat, solvent, or moisture. Japanese Unexamined Patent Publication (KOKAI) No. H9-320758 discloses a patterning method using a mask for vapor deposition. However, it causes poor adhesion between a substrate and a mask, resulting in a blur due to migration of an evaporating material to a clearance at the poor adhesion portion. In addition, if a fine pattern is formed by the mask, an accurate pattern can not be formed because of the problems, such as deflection of the mask, due to insufficient strength of the mask.
A laser processing using excimer laser or YAG laser is considered (Japanese Unexamined Patent Publication (KOKAI) No. H9-50888). However, this method causes poor productivity due to a difference of workability between places with or without the organic material film beneath the cathode film. The laser processing also causes a short-circuiting of an anode with a cathode due to formation of burrs around the cathode line edges by the laser processing. Further, an organic material layer around the laser-irradiated portion suffers from a damage due to laser light.
To solve this problem, a so-called shadow mask method is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. H5-275172 (corresponding to U.S. Pat. No. 5,276,380). In this method, barrier walls in a stripe shape with a height of several to several tens xcexcm are formed parallel to each other on the ITO-patterned substrate. Patterning of a cathode or an organic EL medium is performed by evaporating a material for the cathode or the organic EL medium from the direction perpendicular to the barrier walls or oblique to the substrate surface.
Japanese Unexamined Patent Publication (KOKAI) No. H8-315981 discloses another method, in which a series of electrically insulating barrier walls is formed on a surface of a substrate bearing ITO lines. The lines of the barrier walls are orthogonal to the ITO lines. The barrier wall has on its top an overhanging portion protruding in the direction parallel to the substrate surface, i.e. the barrier wall has an inversely tapered cross section. After the formation of the barrier walls, an organic EL medium layer and a cathode are formed in this order. Each electrode of the cathode is separated by the overhangs of the barrier walls and the electrodes at both sides of the barrier wall are electrically insulated from each other. In this method, short-circuiting of an anode of ITO and a cathode is prevented by means where organic EL medium vapor flows around under the overhang of the barrier wall in a process of a light-emission layer formation, and in the following cathode formation, metal vapor flows around under the overhang of the barrier wall less than the flow of the organic EL medium vapor. Also disclosed is an insulation film disposed beneath the barrier wall, so that short-circuiting of an anode and a cathode, specifically short-circuiting at a cathode edge, is avoided.
The above-cited method performs fine processing of the cathode and the organic EL layer. However, if a photoresist for lift-off is used in the reverse-tapered barrier wall, the barrier wall is liable to peel-off, which causes short-circuiting between the adjacent upper electrodes and results in unsatisfactory patterning of the upper electrodes.
The present invention has been made in view of the above, and an object of the invention is to provide an organic thin-film light-emitting device comprising barrier walls as a shadow mask, in which peeling-off of the walls at their ends is prevented by a structure without free ends, and upper electrodes with any desired pattern can be reliably fabricated.
In order to attain the object, the invention provides an organic thin-film light-emitting device having an image display array formed of a plurality of light-emitting areas. The organic thin-film light-emitting device comprises a substrate, a plurality of first electrodes arranged parallel on the substrate, an EL layer of an organic material formed on an electrode pattern of the first electrodes, a plurality of second electrodes formed on the EL layer, and an electrically insulating barrier wall arrangement including a barrier wall line portion and barrier wall end portions. The barrier wall line portion includes a plurality of barrier wall lines disposed at gaps in an electrode pattern of the second electrodes, and the ends of the barrier wall lines are connected to the barrier wall end portions. No free end is left in the barrier wall lines.
Advantageously, the barrier wall arrangement comprises an insulating film and a photoresist for lift-off formed on the insulating film.
Advantageously, the width of the barrier wall end portion is wider than the width of the gap in the electrode pattern of the second electrodes and wide enough to prevent peeling-off of the barrier wall lines at their ends.